Polytetrafluoroethylene fine powder and process for producing the same

ABSTRACT

Polytetrafluoroethylene fine powders which have an average molecular weight of not less than 5,000,000, preferably not less than 5,500,000, an amorphous index of not more than 0.1, preferably not more than 0.09, a number average primary particle size of 0.1 to 0.4 micron, preferably 0.15 to 0.38 micron, and a sharp endothermic peak at a temperature between 343° C. and 350° C. on the melting diagram by a differential scanning calorimeter, said sharp peak having an endothermic ratio of not more than 0.3, preferably not more than 0.27, and a half value width of the endothermic peak of not more than 8 degrees, preferably not more than 6 degrees, and a process for producing the polytetrafluoroethylene fine powders. The polytetrafluoroethylene fine powders have an excellent stretchability and are useful for electric wire coating, small diameter tubes, large diameter tubes, sealing tapes and the like.

The present invention relates to novel polytetrafluoroethylene finepowders (hereinafter, referred to as "PTFE fine powders") and a processfor producing the same.

PTFE fine powders are used for electric wire coating, small diametertubes, large diameter tubes, sealing tapes and the like, and there arealready many studies on the relationship between the PTFE fine powdersand their processability. For example, PTFE fine powders suitable forelectric wire coatings and small diameter tubes are such that they canbe extruded at a high reduction ratio (a ratio of the cross section ofthe cylinder of an extruder to that of the nozzle of the extruder). SuchPTFE fine powders are obtained almost satisfactorily by the techniquesdisclosed in U.S. Pat. No. 3,142,665, and U.S. Ser. No. 682,593. Forlarge diameter tubes, PTFE fine powders that can be extruded at a lowreduction ratio are suitable PTFE fine powders suitable for sealingtapes (unsintered tapes) are such that the tapes are hardly broken,corrugated or devitrified by the shearing force applied thereto duringhigh-speed calendering. For such purpose, the PTFE fine powders that canbe extruded at a high reduction ratio are not suitable. In this way,various types of PTFE fine powder having properties suitable for each ofthe various uses and processing conditions have been developed and usedeffectively.

Recently, a growing interest has been taken in the special applicationof PTFE to porous materials and some production methods have alreadybeen reported. For example, one typical example of the methods isdisclosed in U.S. Pat. No. 3,962,153. This U.S. patent describes aprocess for producing porous materials having a matrix tensile strengthof 514 kg/cm² or more by stretching a paste extrudate of PTFE finepowder, as it is unsintered, at a high speed of 10%/sec or more. Thisprocess is accomplished based on the discovery that the unsintered pasteextrudate has a very unique property of beig highly stretchable, withoutbeing broken, at a high temperature and a very high stretching rate. Thethus stretched PTFE is a porous material having a high tensile strengthin the direction of stretching and a low apparent density, and it isused for various purposes as it is or after sintering.

An object of the present invention is to provide novel PTFE fine powdershaving excellent stretchability. Another object of the invention is toprovide novel PTFE fine powders which are highly crystalline and have ahigh molecular weight. A further object of the invention is to provide aprocess for producing PTFE fine powders which are highly crystalline andhave excellent stretchability and a high molecular weight. These andother objects of the invention will be apparent from the followingdisclosure.

As is disclosed in U.S. Pat. No. 3,962,153, when the unsintered pasteextrudate of PTFE is stretched, the higher the stretching temperature,the less likely it is broken, and the higher the stretching rate, thegreater is its stretchability without being broken. Thus, a stretchingrate as high as 5,000%/sec to 4,000%/sec is occasionally applied. Thestretchability (a property to be stretched without break) varies withthe kind of starting PTFE fine powders. For example, a modified PTFEfine powder obtained by the process disclosed in U.S. Pat. No. 3,142,665is poor in stretchability, and it is therefore necessary to stretch thepowder at a higher speed and a higher temperature in order to obtainporous materials. Furthermore, it is known that the stretchability ofPTFE fine powder tends to become better as the degree of crystallinityof the polymer constituting the fine powder becomes higher, and that thestretchability tends to be improved when the fine powder is previouslysubjected to heat treatment at a temperature between 200° C. and themelting point of the fine powder.

The PTFE fine powders according to the present invention arecharacterized in that they have an average molecular weight of not lessthan 5,000,000, more preferably not less than 5,500,000, an amorphousindex (hereinafter, referred to as "A.I.") of not more than 0.1, morepreferably not more than 0.09, and a number average primary particlesize of 0.10 to 0.40 micron, more preferably 0.15 to 0.38 micron; andthat, when their melting points are measured by a differential scanningcalorimeter (hereinafter, referred to as "DSC"), they show meltingdiagrams wherein a sharp endothermic peak is observed at a temperaturebetween 343° C. and 350° C., said peak having an endothermic ratio ofnot more than 0.3, preferably not more than 0.27, and a half value widthof the endothermic peak of not more than 8 degrees, preferably not morethan 6 degrees.

In the present invention, the average molecular weight (Mn) iscalculated by the following equation from the specific gravity(hereinafter, referred to as "S.G.") of the polymer:

    log.sub.10 Mn=28.524-9.967×S.G.

According to this equation, for example, the average molecular weight of5,000,000 corresponds to S.G. of 2.19. In other words, the term "averagemolecular weight of not less than 5,000,000" used in the presentspecification has the same meaning as "S.G. of not more than 2.19".Likewise, "the average molecular weight of not less than 5,500,000" isthe same as "S.G. of not more than 2.185"

The S.G. of the polymer can be measured in the following manner: Asample of the powder (5 g) is compressed in a mold having a circularsection of a diameter of 32 mm under an atmospheric temperature of 23°to 25° C. and a pressure of 200 kg/cm², and the sample thus treated istaken out from the mold and sintered in an air furnace at a temperatureof 380° C. for 30 minutes, cooled to 300° C. at a cooling rate of 70°C./hour, and then the sintered sample is taken from the furnace andallowed to cool at room temperature. The weight of the sample thusobtained is measured. .[.The S.G. is shown by the ratio of the weight ofthe sample in air to that of water having the same volume at 23° C..]..Iadd.The S.G. is shown by the value obtained by multiplying the ratioof the weight of the sample in air to that of water having the samevolume at 23° C. by 0.9822 and then adding 0.04864 to the resultingvalue. .Iaddend.

The A.I. in the present specification means the value calculated bydividing the absorbance of the polymer at a wave number of 778 cm⁻¹ bythe absorbance at a wave number of 2367 cm⁻¹ in infrared spectrum.

The melting test by a DSC is carried out as follows:

Ten milligrams of the unsintered PTFE fine powders are accuratelyweighed and placed in an aluminum pan, and the measurement for meltingof crystal at the melting point is carried out using a highly sensitiveDSC (DSC-II type, made by Perkin-Elmer Co., Ltd.). At this time, anendothermic peak owing to melting is recorded on the chart in proportionto heat of fusion at the melting point. The rate of rise of temperatureshould be accurately adjusted to 20° C./minute after the temperaturebecomes at least 80° C. lower than that at which the endothermic peakowing to melting appears. As is well known, the temperature at whichPTFE crystals give an endothermic peak on the DSC chart and the shape ofthe peak are influenced by the rate of temperature-rise during themeasurement (refer to, for example, Appl. Polymer Symposia, No. 2,101-109 (1966)). The temperature of the endothermic peak and the shapeof the peak thus measured delicately reflect the state of unsinteredPTFE fine powder crystals.

According to the above measurement on DSC, PTFE has an endothermic peakat around of 347° C. The accompanying FIG. 1 shows the endothermic curveof the PTFE fine powder produced in Example 2 as described hereinafter,which is measured by DSC. It is clear from FIG. 1 that the powder has anendothermic peak at about 346° C. (619° K.).

The terms "endothermic ratio" and "half value width of the endothermicpeak" used in the present specification are defined as follows:

In FIG. 1, a vertical broken line is drawn downward from the peak (A)and the cross point thereof with the base line (L) is named as (B).Besides, a line parallel with the vertical line is drawn at atemperature 10° C. lower than the temperature of top of the peak [i.e.346° C. (619° K.)] and the cross points thereof with the endothermiccurve and with the base line (L) are named as (C) and (D), respectively.Then, the value of CD/AB is defined as the "endothermic ratio".Moreover, at the middle point (F) of the vertical line AB, a lineparallel with the abscissa axis is drawn and the cross points thereofwith the endothermic curve are named as (E) and (G), respectively. Thedistance EG, i.e. the width of the peak at half of the height from thebase line of the endothermic curve, is defined as "half value width ofthe endothermic peak", which unit is shown in degrees.

The meanings of the above-mentioned properties of the present PTFE finepowders are explained below.

The PTFE fine powders of the present invention have an average molecularweight of not less than 5,000,000, more preferably not less than5,500,000, usually in the range of 5,000,000 to 20,000,000. When themolecular weight is lower than 5,000,000, the stretchability of the finepowders is not good even though other requirements are satisfied. Thevalue of A.I. should be not more than 0.1, more preferably not more than0.09, usually in the range of 0.02 to 0.1. Higher A.I. values indicatethe presence of more incomplete crystals of PTFE. When the value is morethan 0.1, the fine powders become poorer in stretchability. The finepowders having a number average primary particle size of less than 0.1micron are not suitable for processing, because they induce rise of apressure under which the PTFE paste is extruded. On the other hand, thefine powders having a number average primary particle size of more than0.4 micron are also undesirable because the extrudate shows a lowstrength.

The PTFE fine powders of the present invention have an endothermic peakof melting of the crystals measured by DSC at a temperature between 343°C. and 350° C. When a PTFE fine powder has the endothermic peak at atemperature out of the above range, it is not included in the finepowders of the present invention. In most fine powders of the presentinvention, the endothermic peak appears at a range of 345° to 349° C.Some fine powders occasionally show two peaks, and in such a case, thehigher peak is defined as endothermic peak. The higher the molecularweight of the polymer, the higher the temperature of the endothermicpeak. Taking into consideration this fact, the PTFE fine powders of thepresent invention are among the highest molecular weight polymers.Moreover, the present PTFE fine powders are characteristic in that theendothermic peak measured by DSC is very sharp. That is, the sharp peakhas an endothermic ratio of not more than 0.3 and a half value width ofthe endothermic peak of not more than 8 degrees. When a PTFE fine powderdoes not satisfy the conditions of the endothermic ratio and the halfvalue width of the endothermic peak as mentioned above, it does not showgood stretchability of tape, even though the powder has an endothermicpeak at 343° to 350° C. and other properties thereof are within therequired ranges. These relationships found by the present inventorsbetween the DSC chart and the stretchability of paste extrudate are veryinteresting.

FIGS. 1 to 4 show an example of the DSC chart of the PTFE fine powdersin the Examples and reference Examples. All these DSC charts show only aregion between about 310° C. (583° K.) and about 370° C. (643° K.)wherein an endothermic peak appears, the abscissa indicating atemperature (absolute temperature: °K.) and the ordinate indicating aquantity of heat absorbed per unit time (mcal/sec). Vertical brokenlines are drawn passing the endothermic peak and the temperature 10° C.lower than said peak. FIGS. 1 and 2 show the DSC chart of the powdersatisfying the requirements of the present invention. As is clear fromthese charts, the endothermic peak is very sharp and has an endothermicratio of not more than 0.3 and a half value width of the endothermicpeak of not more than 6 degrees. FIGS. 3 and 4 show the DSC chart of thepowder as disclosed in Referenced Examples 1 and 2, and any of them doesnot satisfy either or both of the requirements of the endothermic ratioand the half value width of the endothermic peak. As is clear from thecomparison of FIGS. 1 and 2 of the present fine powders and FIGS. 3 and4 of the powders in reference, in case of the powders of reference, aclear shoulder or another peak appears at a temperature about 10 degreelower than that of the endothermic peak, but on the contrary, in case ofthe powders of the present invention, such a clear shoulder does notappear, which is one of the characteristics of the present PTFE finepowders.

FIGS. 5 to 8 show the DSC chart of various commercially available PTFEfine powders. In these commercial products, neither shoulder nor otherpeak appears at a temperature about 10 degrees lower than that of theendothermic peak, and the endothermic ratio is low in all products. But,all these products have a half value width of the endothermic peak ofmore than 6 degrees, and in case of FIGS. 5 and 8, the temperature ofthe endothermic peak is lower than 343° C. (616° K.). Thus, thesecommercial products do not satisfy the requirements in the presentinvention in regard to the above matters, and are all inferior in thestretchability of tape.

Since the PTFE fine powders of the present invention have all theexcellent properties as mentioned above, they are superior to thecommercial fine powders in the mechanical properties of tubes or tapesproduced by extrusion molding thereof, in addition to the superiorstretchability as described above. Particularly, the present PTFE finepowders are characterized by a high resistance to repeated bending.

The PTFE fine powders of the present invention can be produced bypolymerizing tetrafluoroethylene in an aqueous medium in the presence ofan anionic surface active agent, dispersion stabilizer andpolymerization initiator at a polymerization temperature of 55° to 85°C., with changing the polymerization conditions after the initiation ofthe polymerization and after the polymer in an amount of at least 25% byweight, more preferably 30% by weight, of the final yield has beenproduced but before at most 85% by weight, preferably at most 80% byweight, of the final yield of the polymer has been produced, saidchanging of polymerization reaction being carried out by at least onemethod of adding an alkali or a radical scavenger or both to thereaction system, and lowering the polymerization temperature at least 5°C.

As the anionic surface active agent, there may be exemplifiedwater-soluble fluorine-containing surface active agents, for example,compounds of the formulae: X(CF₂)_(n) COOH (wherein X is hydrogen,chlorine or fluorine atom and n is an integer of 6 to 12) or Cl(CF₂CFCl)_(n) CF₂ COOH (wherein n is an integer of 2 to 6), and saltsthereof. These surfactants may be used in an amount of about 0.05 to0.5% by weight based on the weight of the aqueous medium.

As the polymerization initiator, a water-soluble persulfate (e.g.ammonium persulfate, potassium persulfate) or a mixture thereof with awater-soluble aliphatic dibasic carboxylic acid perioxide (e.g.disuccinic acid peroxide, diglutaric acid peroxide) is generally used.The polymerization initiator may preferably be used in a relatively lowconcentration, because such a low concentration gives a desirable effecton the stretchability of the extrudate and further it is difficult toproduce a polymer having a high molecular weight when a too highconcentration of the initiator is used. That is, the water-solublepersulfate is preferably used in a concentration of 0.002% by weight orless, more preferably 0.001% by weight or less, practically in the rangeof 0.0001 to 0.002% by weight, regardless of whether it is used alone orin a combination of the aliphatic dibasic carboxylic acid. The aliphaticdibasic carboxylic acid peroxide is preferably used in a concentrationof 0.01% by weight or less.

Suitable examples of the dispersion stabilizer are hydrocarbons having12 or more carbon atoms which are present in the liquid form under thepolymerization condition.

All the compounds described above are generally used in thepolymerization of tetrafluoroethylene and are easily available.

The polymerization reaction in the present invention should be carriedout at a polymerization temperature of 55° to 85° C. Higherpolymerization temperature than 85° C. induces too rapid decompositionof the catalyst and hence is not suitable for the polymerization in thepresent invention wherein a small amount of a catalyst is used. On theother hand, when the polymerization temperature is lower than 55° C.,the decomposition of the catalyst is too slow, and hence, thepolymerization reaction does not proceed at a sufficient rate. Thepolymerization can be carried out under a pressure of 5 to 20 kg/cm²,preferably 5 to 10 kg/cm².

As mentioned above, the polymerization condition is changed after theinitiation of polymerization by any one of the methods as mentionedabove. When the polymerization condition is not changed, the producedPTFE has a too low molecular weight or a too broad endothermic peak inmelting of the crystal measured by DSC, and the desired PTFE fine powderhaving an excellent stretchability can not be obtained. These methodsfor changing the polymerization condition are explained below in moredetail.

One of the methods comprises adding an alkali to the reaction system.Suitable examples of the alkali are ammonium hydroxide, sodiumhydroxide, or potassium hydroxide, which are preferably used in anaqueous solution. The alkali is used in such an amount that thepolymerization system is kept in an alkaline side, particularly in a pHrange of 8 to 10. Thus, the amount of the alkali should be determined inaccordance with the pH value of the reaction system, the kinds andamounts of the used polymerization initiator, use of buffer, or thelike, and the suitable amount thereof will readily be determinedexperientially or by a provisional test. For example, when a persulfatesuch as ammonium persulfate or potassium persulfate is used as thepolymerization initiator, the pH value of the polymerization system isusually rapidly lowered to about 3 to 4, while the pH value before theinitiation of the polymerization reaction lies in about 6 to 7, butaccording to the present invention, the pH value is changed to 8 to 10by addition of an alkali. When an alkali is added, the rate ofpolymerization is lowered in comparison with the case of no addition ofalkali, but the lowering of the rate of polymerization is slower incomparison with that which occurs at the middle of the reaction step orby addition of a radical scavenger. That is, when an alkali is added tothe polymerization system, the pH value in the system is rapidlyincreased to 8 to 10, but the reduction of the rate of polymerization isgradually attained.

The second method for changing the polymerization condition comprisesadding a radical scavenger which is effective on radicals in the aqueousphase. Suitable examples of the radical scavenger are ammoniumthiocyanate (NH₄ SCN), potassium thiocyanate (KSCN), sodium thiocyanate(NaSCN), cupric chloride (CuCl₂), or the like. Addition of an excessamount of the radical scavenger unfavorably deteriorates the stabilityof the dispersion, and hence, the radical scavenger is preferably usedin an amount of about 2 to 5 times by weight to the amount of thepolymerization initiator in order to obtain the desired PTFE finepowders.

The above alkali and radical scavenger may be used either alone ortogether.

The third method for changing the polymerization condition compriseslowering the polymerization temperature at least 5° C., preferably atleast 10° C. This can be carried out by changing the pre-set temperatureof an automatic temperature controller provided for controlling thetemperature of the reaction vessel. When the temperature of thecontroller is changed, the temperature in the reaction vessel is notimmediately changed but is rather delayed. However, it is enough thatthe change of temperature is completed before the polymer of about 80%by weight of the final yield has been produced, and hence, it is not sodifficult to control the temperature. When the change of the reactiontemperature is less than 5° C. the desired PTFE fine powder havingexcellent properties can not be obtained. On the other hand, when thetemperature is changed more than 30° C., the reaction system becomesunstable, which induces unfavorable coagulation of the dispersion.

The change of the polymerization condition as mentioned above should bedone at the time after the polymer of at least 25% by weight, preferablyat least 30% by weight, of the final yield has been produced, but beforethe polymer of at most 85% by weight, preferably at most 80% by weight,of the final yield has been produced. It is known that the emulsionpolymerization of tetrafluoroethylene proceeds in two reaction stages;the first stage wherein the nuclei of the polymer particles is mainlyformed and the second stage wherein the particles mainly grow. When thepolymerization condition is changed at the first stage of thepolymerization which is a nucleus-forming stage, the reaction systembecomes undesirably unstable. On the other hand, when the polymerizationcondition is changed after the polymer of more than 85% by weight of thefinal yield has been produced, the desired PTFE fine powder havingexcellent properties can not be obtained.

Generally, in emulsion polymerization of tetrafluoroethylene, the rateof polymerization increase with the lapse of time, if a polymerizationinitiator is used in a comparatively low concentration and the reactiontemperature is fixed so that the half life time of the polymerizationinitiator becomes comparatively long, and further, if the temperatureand monomer pressure are maintained constant during the polymerization.This may occur due to the following reasons: the polymer radicalproduced by the reaction of tetrafluoroethylene with a radical resultingfrom decomposition of the initiator is comparatively hardly deactivatedthroughout the polymerization wherein the particles of the polymer areformed, and the particles adsorb the radicals successively produced bythe same mechanism, as a result of which the number of the radical inthe whole reaction increases with the lapse of time.

On the other hand, the rate of decomposition of the initiator is largelyinfluenced by the pH value and temperature of the system, and it becomeslarger with lowering of the pH and with rise of the temperature. Inemulsion polymerization of tetrafluoroethylene, when the polymerizationis performed at a constant temperature and under a constant monomerpressure using the above-mentioned polymerization initiator, the pH ofthe system is lowered gradually unless any pH regulator is added, andthe rate of decomposition of the initiator tends to become large. As aresult, the overall rate of polymerization of tetrafluoroethylene isaccelerated.

Addition of alkali and/or lowering of polymerization temperature in thecourse of polymerization induce slowdown of the rate of decomposition ofthe undecomposed initiator remained in the system, and limit the numberof the polymer radicals successively produced. As a result, the rate ofpolymerization is slowed down compared with the case when thepolymerization condition is not changed. Ammonium thiocyanate, potassiumthiocyanate, sodium thiocyanate and cupric chloride act as a radicalscavenger in an aqueous phase. When the radical scavenger is added, theradicals produced after the addition thereof are arrested anddeactivated, as a result of which the rate of polymerization is sloweddown. Accordingly, the radical scavenger is also effective forinhibiting the successive production of polymer radicals.

The aqueous dispersion of polymer thus obtained is subsequentlycoagulated, washed and dried by an ordinary after-treatment procedure,and thereby, the desired PTFE fine powders are produced.

The PTFE fine powders of the present invention thus obtained achieve ahigh molecular weight and a high crystallinity. Further, the PTFE seemsto have a narrow distribution of molecular weight, because the radicalssuccessively produced in the course of polymerization are limited. Infact, the melting diagram of the present PTFE fine powders on the DSCchart suggests the narrow distribution of molecular weight.

The PTFE fine powders according to the present invention can besubjected to various after-treatments, for example, to heat treatment at300° C. or lower, and the kneading or pulverizing treatment as disclosedin Japanese Patent Publication No. 4657/1971. By these treatments, themelting diagram is changed very little and the stretchability isgenerally improved.

The PTFE fine powders of the present invention may contain fillers, suchas powdered glass fiber, carbon powder, graphite powder, inorganicpigment powder or the like. In producing the PTFE fine powderscontaining such fillers, it is desirable to blend the fillers and thepolymer at the coagulation step of the polymer dispersion obtained bypolymerization. The PTFE fine powders thus obtained have also a goodstretchability.

The present invention will be illustrated by the following examples. Inthe examples, percentage indicating a concentration and a weight ratioare by weight, unless otherwise stated.

The extrusion test and stretching test are carried out as follows:

Extrusion test:

100 parts by weight of the PTFE fine powder and 20 parts by weight of aliquid lubricant (Isopar E, a tradename of Esso Standard Oil Co.) aremixed at room temperature, and the mixture is stored for 12 to 24 hoursin a tightly sealed container. The mixture is then extruded intofilament at room temperature at a ram speed of 20 mm/min on an extrusionmold (inside diameter of cylinder, 25.4 mm; die angle, 30°; nozzlediameter, 2.54 mm; nozzle length, 7 mm). In this extrusion test, anextrusion pressure at the equilibrium state is recorded. The filament isthen dried.

Stretching test:

A test sample for stretching test is prepared by cutting the filamentobtained in the extrusion test. The sample is stretched at 310° C., witha distance between chucks of 50 mm, to 20 folds as long as its originallength, at a varying stretching rate of 100 %/sec, 1000 %/sec and 1000%/sec. The sample which can be stretched to 20 folds as long at astretching rate of 100 %/sec is not also broken at the other two rates,and therefore it may be judged to have the highest stretchability. Onthe other hand, the sample which is broken at a stretching rate of 10000%/sec is also broken at the other two rates, and therefore it may bejudged to have the lowest stretchability.

EXAMPLE 1

To a 3-liter glass-lined autoclave equipped with a stirrer were added1.5 liter of an ion- and oxygen-free water, 60 g of paraffin wax (m.p.56° C.) and 2 g of ammonium perfluorooctanoate. The atmosphere in theautoclave was replaced with nitrogen gas several times at 70° C., andtetrafluoroethylene (hereinafter, referred to as "TFE") was chargedunder pressure until the inner pressure became 8.0 kg/cm² G. Thereafter,5 mg of ammonium persulfate (hereinafter, referred to as "APS") wasadded and polymerization was stated. The pressure in the autoclave beganto drop as the polymerization started. When the pressure dropped to 7.0kg/cm² G, TFE was charged again under pressure until the pressure becameto 8.0 kg/cm² G. Thereafter, this cycle of pressure-drop and TFE-chargewas repeated with the process of the reaction, and the polymerizationwas stopped 5 hours and 10 minutes after the initiation of thepolymerization. In the course of the polymerization, when 0.8 ml of 28%aqueous ammonia was added to the reaction system about 3 hours after theinitiation of polymerization, the apparent rate of polymerization whichslowed a tendency to increase prior to addition of the aqueous ammoniashowed a tendency to slightly decrease. The pH value of the reactionsystem was checked by sampling the reaction mixture in the system. As aresult, the pH value of the system was 4.1 before the addition of theaqueous ammonia, but became 10 when the aqueous ammonia was added, andthe pH value was kept at about 10 without lowering until thepolymerization finished. Besides, it ws confirmed by sampling that thepolymer of 51% by weight of the final yield was produced before theaddition of the aqueous ammonia. The average rate of polymerization was47 g/liter per hour throughout the whole polymerization period. The PTFEdispersion thus obtained had a concentration of 20.9% and an averageparticle size of 0.29 micron. This dispersion was coagulated, washed anddried at 120° C. for 16 hours to obtain the PTFE fine powder. Thispowder had an S.G. of 2.177, a molecular weight of 6,740,000 and showedan A.I. value of 0.089 by infrared absorption spectrum. The meltingdiagram in the vicinity of the melting point of this powder measured byDSC is shown in FIG. 2. This diagram has a sharp peak at 347° C. (620°K.) and a weak shoulder at 337° C. (610° K.), and the endothermic ratiois 0.21 and the half value width of the endothermic peak is 4.3 degrees.

Fifty grams of ths powder was mixed with 10 g of a liquid lubricant(Isopar E) and the mixture was stored in a tightly sealed container for15 hours. Thereafter, the mixture was extruded into filament at a ramspeed of 20 mm/min on a mold (inside diameter of ram, 25.4 mm; dieangle, 30°; inside diameter of land, 2.54 mm; land length, 7 mm). Theextrusion pressure at the equilibrium state was 182 kg/cm². The filamentextruded was dried and, when subjected to the stretching test at 310°C., it could be stretched to 20 folds as long at a stretching rate of100 %/sec to obtain a product having a uniform appearance.

EXAMPLE 2 to 4

Using the same equipment as in Example 1, polymerization was carried outin the same manner as in Example 1 except that the polymerizationtemperature was 65° C. in place of 70° C. and that aqueous ammonia wasadded at the times shown in Table 1. The properties of the resultingaqueous PTFE dispersions and PTFE powders are shown in Table 1. Themelting diagram by DSC of the powder of Example 2 is shown in FIG. 1,and the powders of Examples 3 and 4 showed almost the same digram as inFIG. 2.

                  TABLE 1                                                         ______________________________________                                                                            Con-                                                                 Polymeriza-                                                                            centration                                     Time of addi-         tion rate                                                                              of dispersion                             Ex.  tion of aque-                                                                            Polymeriza-                                                                              (g/liter per                                                                           (% by                                     No.  ous ammonia*                                                                             tion time  hour)    weight)                                   ______________________________________                                        2    30         9 hr. 27 min.                                                                            28       20.2                                      3    58         8 hr. 12 min.                                                                            35       22.3                                      4    77         9 hr.  4 min.                                                                            27       20.4                                      ______________________________________                                         *This is shown by % by weight of the polymer based on the final yield         which is produced before the addition of the aqueous ammonia             

Reference Example 1

Using the same equipment and the same procedure as in Example 1,polymerization was started and continued for 6 hours without addingaqueous ammonia in the course of the polymerization. The rate ofpolymerization showed a slow increase, that is, the so-calledacceleration tendency, throughout the whole polymerization period. Theaverage rate of polymerization was 43 g/liter per hour. Besides, the pHvalue of the reaction system was 3.2 at the finish of the polymerizationreaction.

The aqueous PTFE dispersion thus obtained had an average particle sizeof 0.27 micron and a concentration of 18.7%. This dispersion wascoagulated, washed and dried to obtain a powder having properties asshown in Table 2. The melting diagram of this powder by DSC is as shownin FIG. 3 and it has a peak at 347° C. (620° K.) and a clear shoulder at337° C. (610° K.)

The extrusion pressure in paste extrusion of this powder was 181 kg/cm².In the 20-fold stretching test at 310 C., the extruded filament wasbroken at a stretching rate of 100 %/sec and 100 %/sec. When thefilament was stretched at a stretching rate of 10000 %/sec in the abovetest, it showed a surface having numberless, non-uniform cracks.

Reference Example 2

Polymerization was carried out at 90° C. using the same equipment as inExample 1. Required amounts of deionized water, paraffin wax andammonium perchlorooctanoate, and 100 mg of disuccinic acid peroxide (apolymerization initiator, referred to as "DSP" hereinafter) were added.

After replacement with nitrogen gas, TFE was charged under pressureuntil the inner pressure became 8 kg/cm² G, followed by stirring. 10 mgof APS was added 1 hour after the addition of DSP. The polymerizationwas substantially started at the time when APS was added, and a drop inpresence was observed. When the inner pressure dropped to 7 kg/cm² G,TFE was charged under pressure until the pressure became to 8 kg/cm² G.Thereafter, the polymerization was continued while the cycle ofpressure-drop and TFE-charge was repeated until the end ofpolymerization. The aqueous PTFE dispersion after polymerization had aconcentration of 25%. This dispersion was coagulated, washed and driedto obtain a powder having properties as shown in Table 2. The meltingdiagram of this powder by DSC is as shown in FIG. 4 and it has a peak at342° C. (615° K.) and another peak at around 332° C. (605° K.). Theextrusion pressure in paste extrusion of this powder was 143 kg/cm.sup.2, and the extruded filament was broken in the 20-fold stretching testat 10000 %/sec.

EXAMPLE 5

The emulsion polymerization of TFE was carried out in the same manner asin Example 1 except that 6 mg of potassium persulfate was used in placeof APS as the polymerization initiator.

The PTFE dispersion thus produced had a concentration of polymer of20.3%. The properties of the PTFE fine powder are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                             DSC analysis                                                                               Half value                                                                           Extrusion                                            Average                                                                            Temp. of     width of                                                                             pressure in                               Mole-      particle                                                                           endothermic                                                                           Endo-                                                                              endothermic                                                                          paste extru-                     Example  cular      size peak    thermic                                                                            peak   sion test                        No.   S.G.                                                                             weight A.I.                                                                              (μ)                                                                             (°C.)                                                                          ratio                                                                              (degree)                                                                             (kg/cm.sup.2)                                                                       Stretchability             __________________________________________________________________________    Example 2                                                                           2.174                                                                            7.17 × 10.sup.6                                                                0.084                                                                             0.32 346     0.19 5.3    160   Stretchable at                                                                100%/sec                   Example 3                                                                           2.173                                                                            7.24 × 10.sup.6                                                                0.089                                                                             0.34 347     0.22 4.2    156   The same above             Example 4                                                                           2.180                                                                            6.18 × 10.sup.6                                                                0.071                                                                             0.27 346     0.28 4.3    176   The same above             Example 5                                                                           2.180                                                                            6.18 × 10.sup.6                                                                0.091                                                                             0.27 347     0.25 4.4    185   The same above             Reference                                                                     Example 1                                                                           2.190                                                                            5.00 × 10.sup.6                                                                0.067                                                                             0.27 347     0.31 4.5    181   Broken at 1000%/sec        Reference                                                                     Example 2                                                                           2.205                                                                            3.50 × 10.sup.6                                                                0.068                                                                             0.28 342     0.52 15     143   Broken at                  __________________________________________________________________________                                                       10000%/sec             

EXAMPLE 6

Emulsion polymerization of TFE was carried out at 70° C. using a1.5-liter glass-made pressure vessel. The amounts of ion-free water,paraffin wax and ammonium perfluorooctanoate were half of those as inExample 1, but the amount of APS was 5 mg. The pressure of TFE was keptat 8 kg/cm² G to 7 kg/cm² G in the same manner as in Exampmle 1. Whenthe yield of polymer reached 54% of the final yield, 20 mg of ammoniumthiocyanate (NH₄ SCN) was added. Thereafter, the rate of polymerizationshowed a tendency to decrease. The total polymerization time was 5 hoursand 17 minutes, and the average rate of polymerization was 48 g/literper hour. The PTFE dispersion had a concentration of 20% an averageparticle size of 0.23 micron, an S.G. of 2,186, a molecular weight of5,370,000 and an A.I. value of 0.073. The melting diagram by DSC wasalmost the same as in FIG. 2, and it had a sharp peak at 347° C. (620°K. and no shoulder at a lower temperature region, and the half valuewidth of the endothermic peak was 4.2 degrees and the endothermic ratiowas 0.2. The pressure in paste extrusion was 145 kg/cm², and theextruded filament could be stretched to 20 folds as long at a stretchingrate of 100%/sec. The stretched product was a satisfactory porousproduct having a uniform appearance.

EXAMPLE 7

Example 6 was repeated except that 20 mg of potassium thiocyanate wasused in place of ammonium thiocyanate.

The resulting polymer dispersion had a concentration of 19.6%, and thepowder produced therefrom had an average particle size of 0.25 micron,an S.G. of 2.185, a molecular weight of 5,600,000 and an A.I. of 0.075.The power had a melting point by DSC of 347° C. (620° K.), anendothermic ratio of 0.19 and a half value width of the endothermic peakof 4.2 degrees.

The pressure in paste extrusion was 142 kg/cm², and the extrudedfilament could be stretched to 20 folds as long at a stretching rate of100%/sec to obtain a satisfactory porous product.

Reference Example 3

Example 6 was repeated except that the amount of APS (polymerizationinitiator) was increased to 25 mg.

The resulting polymer dispersion had a concentration of 24.5%, and thepowder produced therefrom had an average particle size of 0.21 micron,an S.G. of 2,200, a molecular weight of 3,900,000 and an A.I. of 0.101.The powder showed a melting point by DSC of 347° C. (620° K.), anendothermic ratio of 0.4 and a half value width of the endothermic peakof 5 degrees.

The pressure in paste extrusion was 150 kg/cm², and the extrudedfilament was broken by stretching at a rate of 100%/sec.

EXAMPLE 8

In the same manner as in Reference Example 1, the polymerization wasstarted except that the temperature in the system was fixed at 70° C.When the yield of the polymer reached 60-65% of the final yield, thetemperature in the system was dropped from 70° C. to 60° C., and thepolymerization was continued at 60° C., until the end thereof. It wasconfirmed that the polymerization rate at the time when the yield of thepolymer reached to 70% of the final yield was about 15% lower than whenthe polymerization was continued at 70° C. to obtain the same yield ofthe polymer.

The resulting PTFE dispersion has a concentration of 18.5%, and thepowder obtained therefrom had an average particle size of 0.29 micron,an S.G. of 2.175, a molecular weight of 7,000,000 and an A.I. of 0.086.The melting diagram of the powder by DSC showed a peak at 347° C. (620°K.), and the endothermic ratio was 0.17 and the half value width of theendothermic peak was 4 degrees. The extrusion pressure in pasteextrusion was 190 kg/cm², and the extruded filament could be stretchedat a stretching rate of 100%/sec.

Reference Example 4

With respect to various commercially available PTEF fine powders asshown in Table 3, there were measured the molecular weight, A.I. value,and melting characteristics by DSC, and further, they were subjected tothe paste extrusion test and the stretching test. The results are shownin Table 3.

Besides, the melting diagrams of these commercial products by DSC areshown in FIGS. 5 to 8, wherein FIG. 5 is of Hostaflon VP-22, FIG. 6 isof Fluon CD-1, FIG. 7 is of Soreflon 6-20, and FIG. 8 is of Teflon 6J.

                                      TABLE 3                                     __________________________________________________________________________                              DSC analysis                                                                              Half value                                                                            Extrusion                                            Average                                                                            Temp. of    width of                                                                              pressure in                                Mole-     particle                                                                           endothermic                                                                          Endo-                                                                              endothermic                                                                           paste extru-                               cular     size peak   thermic                                                                            peak    sion test                       Powder S.G.                                                                              weight                                                                              A.I.                                                                              (μ)                                                                             (°C.)                                                                         ratio                                                                              (degree)                                                                              (kg/cm.sup.2)                                                                       Stretchability            __________________________________________________________________________    Hostaflon                                                                     VP-22  2.229                                                                             2.0 × 10.sup.6                                                                0.131                                                                             0.26.sup.(a)                                                                       341    --   12.0    180   Broken at 1000%/sec       Fluon CD-1                                                                           2.223                                                                             2.3 × 10.sup.6                                                                0.078                                                                             0.20.sup.(a)                                                                       340.5  0.16 10.5    185   "                         Fluon                                                                         CD-123 2.185                                                                             5.6 × 10.sup.6                                                                0.067                                                                             --   345    0.33 8.5     172   Broken at 100%/sec        Sereflon                                                                      6-20   2.183                                                                             5.8 × 10.sup.6                                                                0.087                                                                             --   343    0.29 10.5    192   "                         Teflon 6J                                                                            2.213                                                                             2.9 × 10.sup.6                                                                0.091                                                                             0.24.sup.(a)                                                                       340    --   10.0    111   Broken at 1000%/sec       Teflon 6CJ                                                                           2.178                                                                             6.5 × 10.sup.6                                                                0.139                                                                             0.22.sup.(a)                                                                       347    0.22 6.0     107   "                         Polyflon                                                                      F-101  2.186                                                                             5.6 × 10.sup.6                                                                0.122                                                                             0.41 340.5  --   12.0    181   "                         Polyflon                                                                      F-104E 2.206                                                                             3.8 × 10.sup.6                                                                0.094                                                                             0.25 344    0.41 9.5     154   Broken at                 __________________________________________________________________________                                                        100%/sec                   [[Note]:                                                                      .sup.(a) The powder was irradiated with radiant rays of 5 × 10.sup.     rad in order to deagglomerate the powder, and then the particle size          thereof was measured by an electron microscope.                          

What is claimed is:
 1. A polytetrafluoroethylene fine powder which hasan average molecular weight of not less than 5,000,000, an amorphousindex of not more than 0.1 and a sharp endothermic peak at a temperaturebetween 343° C. and 350° C. on the melting diagram by a differentialscanning calorimeter, said sharp peak having an endothermic ratio of notmore than 0.3 and a half value width of the endothermic peak of not morethan 8 degrees.
 2. The polytetrafluoroethylene fine powder according toclaim 1, wherein the average molecular weight is not less than5,500,000.
 3. The polytetrafluoroethylene fine powder according to claim1, wherein the endothermic peak on the melting diagram by a differentialscanning calorimeter appears in the region of 345° to 349° C.
 4. Thepolytetrafluoroethylene fine powder according to claim 1, wherein theendothermic ratio is not more than 0.27.
 5. The polytetrafluoroethylenefine powder according to claim 1, wherein the half value width of theendothermic peak is not more than 6 degrees.
 6. In a process forproducing a polytetrafluoroethylene fine powder, which has an averagemolecular weight of not less than 5,000,000, an amorphous index of notmore than 0.1 and a sharp endothermic peak at a temperature between 343°C. and 350° C. on the melting diagram by a differential scanningcalorimeter, said sharp peak having an endothermic ratio of not morethan 0.3 and a half value width of the endothermic peak of not more than8 degrees, the polymerization reaction at a polymerization temperatureof 55° to 86° C. using a water-soluble persulfate as the polymerizationinitiator in a concentration of 0.002% by weight or less, and changingthe polymerization condition after the initiation of polymerization andafter the polymer in an amount of at least 25% by weight of the finalyield is produced but before at most 85% by weight of the final yield ofthe polymer is produced, said changing of the polymerization conditionbeing carried out by adding either or both of an alkali and a radicalscavenger to the polymerization system or by lowering the polymerizationtemperature 5° to 30° C. said alkali being added in such an amount thatthe polymerization system is kept on an alkaline side and said radicalscavenger being added in an amount of about 2 to 5 times by weight ofthe amount of the polymerization initiator.
 7. The process according toclaim 6, wherein the changing of the polymerization condition is carriedout by adding an alkali to the polymerization system.
 8. The processaccording to claim 7, wherein the addition of the alkali is done afterthe polymer in an amount of at least 30% by weight of the final yield isproduced and before at most 80% by weight of the final yield of thepolymer is produced.
 9. The process according to claim 6, wherein thechanging of the polymerization condition is carried out by adding aradical scavenger to the polymerization system in an amount of about 2to 5 times by weight of the amount of the polymerization initiator, saidradical scavenger being effective on radicals in the aqueous phase. 10.The process according to claim 9, wherein the addition of a radicalscavenger is done after the polymer in an amount of at least 30% byweight of the final yield is produced and before at most 80% by weightof the final yield of the polymer is produced.
 11. The process accordingto claim 6, wherein the changing of the polymerization condition iscarried out by lowering the polymerization temperature at least 5° C.,but not more than 30° C.
 12. The process according to claim 11, whereinthe temperature is lowered at least 10° C.